Theory: Molecular Biology / Genetics
1. Jacob-Monod lac Operon Model
Proposed by Francois Jacob and Jacques Monod (1961, Nobel Prize 1965). Operon = cluster of structural genes controlled together by single regulatory region. lac operon: negative regulation (repressor inhibits), inducible (induced by substrate lactose). Components: CAP site → Promoter → Operator → lacZ → lacY → lacA. The lacI gene (with its own promoter) is separate from but near the operon.
2. Negative vs Positive Regulation
Negative regulation (lac repressor): repressor protein INHIBITS transcription by binding operator. Removal of repressor (by inducer) → transcription ON. Positive regulation (CAP): CAP+cAMP ACTIVATES transcription by binding upstream of promoter. Both work together: maximum lac transcription requires BOTH absence of repressor AND presence of active CAP. This gives fine control depending on both lactose and glucose availability.
3. Allolactose — The True Inducer
Lactose itself is NOT the direct inducer. A small amount of beta-galactosidase (constitutively present at very low levels) converts some lactose to allolactose (a structural isomer of lactose). Allolactose is the actual inducer — binds lac repressor, causes conformational change, repressor releases operator. IPTG (isopropyl thiogalactoside): non-metabolisable synthetic inducer used in laboratory to induce lac operon for protein production.
4. Applications of lac Operon in Biotechnology
The lac promoter (Plac) and its control elements are widely used in recombinant DNA technology: Protein expression vectors in E. coli often use Ptac (hybrid Ptrp-Plac) or PT7 promoters, but lac-based vectors are used extensively. IPTG induction: add IPTG → lac repressor inactivated → strong expression of cloned gene. lacZ reporter gene: used as a reporter for detecting gene expression (blue colonies in blue-white screening contain functional lacZ gene).
Frequently Asked Questions
1. What are the components of the lac operon? ⌄
lac operon components: Regulatory gene (lacI): codes for lac repressor protein; constitutively expressed (always on). Promoter (P): RNA polymerase binding site for structural genes. Operator (O): binding site for lac repressor protein. Structural genes: lacZ (beta-galactosidase: breaks lactose → glucose + galactose), lacY (permease: lactose transport), lacA (transacetylase: minor role). CAP site: for catabolite activator protein (positive regulation).
2. What is constitutive expression? ⌄
Constitutive expression means a gene is expressed at a constant, continuous level regardless of environmental conditions — it is always "on." The repressor gene (lacI) is constitutively expressed because E. coli needs to always have repressor available to rapidly shut off the structural genes whenever lactose becomes unavailable. Constitutive expression produces a "housekeeping" level of the repressor protein.
3. What happens in the absence of lactose? ⌄
No lactose: Repressor protein (from constitutively expressed lacI) binds to operator (O). RNA polymerase blocked at operator. Structural genes lacZ, lacY, lacA NOT transcribed. No beta-galactosidase produced. Glucose (not lactose) metabolised if available.
4. What happens when lactose is present? ⌄
Lactose present: Some lactose converted to allolactose (by residual beta-galactosidase). Allolactose = inducer — binds repressor protein → conformational change → repressor cannot bind operator. RNA polymerase proceeds past operator. Structural genes transcribed → mRNA → beta-galactosidase, permease, transacetylase produced. Lactose broken down to glucose + galactose.
5. What is catabolite repression in lac operon? ⌄
Even when lactose is present, if glucose is also present, lac operon is NOT fully induced (catabolite repression). Glucose → high cAMP metabolism → low cAMP. CAP (catabolite activator protein) requires cAMP to bind. Low cAMP → CAP cannot bind CAP site → weak promoter → low transcription. When glucose absent: cAMP high → CAP+cAMP binds CAP site → strong transcription. E. coli prefers glucose; only fully induces lac operon when glucose absent AND lactose present.